1. Technical Field
The present invention relates generally to light emitting diodes, and more particularly relates to controlling a plurality of light emitting diodes.
2. Background Information
Light emitting diodes (“LEDs”) are becoming increasingly popular as light source devices, especially as their cost continues to decrease. Typical applications for LEDs in automobiles include illumination of information display devices, such as liquid crystal diode displays, and illumination of gauges in instrument panel assemblies.
Typical applications utilize a plurality of LEDs to produce the required total light intensity. In general, the LEDs have a light intensity output that is adjustable over a wide range from full intensity to some small fraction of the full intensity, for example, {fraction (1/100)} of the full intensity, in response to an operator or microcomputer command. The range of operating light intensity is commonly referred to as a dimming range or dimming ratio over which the light intensity from LED to LED remains substantially uniform, so that the total light intensity across the plurality of LEDs appears uniform. Since the perceived light intensity from an LED is proportional to its forward current, a uniform intensity requirement translates into a uniform current requirement. However, uniformity of current from LED to LED can be difficult to achieve in most circuits because of variations in supply and LED voltages and circuit impedances.
Both analog and digital methods have been used to adjust the average current through an LED, and, thus, light intensity of an LED. In the analog method, a voltage or current regulating device makes adjustments in a continuous manner so that the current varies from some maximum level to some minimum level within the required dimming range. In the digital method, commonly referred to as pulse width modulation (“PWM”), a voltage or current source adjusts between two levels (for example, between zero and maximum voltage or current) at a rate that is high enough to be perceived by human visual processes as an average intensity proportional to a duty ratio. Digital methods provide dimming over a wide range without the difficulties associated with regulating very low levels of current as in analog methods. Digital methods also allow operating at a level of current that is most efficient for the LED. In addition, since LED color characteristics are a function of current, operation at a specific current level assists in maintaining constant color over the dimming range. Further, digital methods are particularly suited to microcomputer interface and control.
In typical applications, the LEDs in an LED circuit are connected in either a parallel or series configuration. In the parallel configuration, the LEDs are connected in parallel, which in turn are connected to an adjustable supply voltage that is continuously variable for the analog case, or connected to a supply voltage that is supplied via switching techniques for the digital case. Varying the supply voltage in the analog case or using PWM in the digital case adjusts the current through the LEDs. However, it is difficult to maintain a precise and uniform level of current in each of the LEDs because of variations in supply and LED voltages and circuit resistance. In addition, the presence of resistance in the circuit adds to the total circuit power dissipation and reduces efficiency.
In a series configuration, all of the LEDs, a voltage supply, and/or a current limiting or current regulating device are connected in series. The LED current can then be adjusted with continuous voltage or current adjustment in the analog case or via PWM in the digital case. Use of the series configuration assures uniformity of current since all of the LEDs are connected in series and therefore conduct the same level of current. However, since all of the LEDs are connected in series, the failure of any one LED could lead to a total loss of illumination, which could be a safety issue in some applications.